Follows the school activities of second-graders, Harry and Doug, as they participate in a secret mission to celebrate "Charlotte's Web," learn how to make green slime, and put on a skit warning about the dangers of smoking.
It's almost summer vacation and the students in room 2B are having trouble concentrating on their butterfly project, until a frightening-looking new teacher comes to supervise the Suspension Room and Harry is sent there as punishment for unruly behavior.
Miss Mackle's third graders share December holiday traditions --Christmas, Hanukkah, Kwanzaa, Three Kings Day, and Korean New Year-- with each other and with Harry's great grandfather, who has just moved to a nursing home.
When Horrible Harry's pet cat, Googer, sneaks a ride and gets into the school building, third-graders Harry and Doug have to catch him before anyone else does, which means going into some forbidden places.
Mary is always neat and organized, while Harry is horribly messy. But even Mary's carefully planned schedule can't help when things go wrong at her own birthday party. The party is supposed to start with the kids from Room 3B taking an exciting train ride, but when their train is delayed not one . . . not two . . . but three times, Mary's plan is completely ruined! She thinks the only thing that can make her feel better is cup after cup of sugary...
Spurred on by the promise of a mystery prize, Harry and Doug are determined to win the most points on Field Day, but Harry's sneaky tactics annoy their competition, making the two boys targets for revenge.
As the last day of third grade looms, Doug learns that Harry is moving to a new town and wonders how he'll survive fourth grade without him, while Harry wonders about the new neighbors across the street and what they could mean for their friendship.